The present invention relates to a balancing device for a rotating body, in particular for a tool carrier with a tool rotating at high speed.
More specifically, the invention relates to balancing directly executable during rotation of rotating bodies, i.e. without stopping of the body rotation being required for carrying out a manual or mechanical intervention on the balancing masses.
It is known that in metal machining, tools are presently capable of removing material at very high cutting speeds, even without the workpieces being cooled, and that machine tools or machining centers capable of working at 40,000 revolutions per minute (r.p.m.) and even more are becoming increasingly more widespread.
It is clear that under these rotation conditions balancing of the tool together with its tool carrier mounted on the machine spindle becomes of fundamental importance.
The devices commonly used for balancing rotating bodies during rotation of the same, grinding wheels for example, comprise balancing masses disposed in the rotating body and radially and/or angularly shiftable by means of motors, also disposed in the rotating body and driven from the outside of said body. These devices however need some room and a relatively long period of time for achieving a satisfactory balancing. In addition, due to the concerned masses, they are not generally adapted for balancing of bodies rotating at high speed, beyond 15,000 r.p.m., for example.
Also known are balancing devices comprising two balancing rings, disposed in axial side by side relationship and coaxial with each other on a cylindrical support rigidly connected to the rotating body to be balanced. The rings are susceptible of rotation together with the rotating body but are also free to rotate with respect to said body and each have a known unbalance which is generally the same for both of them, in terms of intensity and angular position.
These rings can be angularly rotated relative to the rotating body, during rotation of the latter, to such a position that the residual unbalance of the rotating body is compensated for by the resultant of the unbalance of said rings.
The relative angular rotation of the rings is controlled from the outside of the rotating body through a magnetic field generated by electric windings disposed on a fixed part and associated with the rings at least along a peripheral portion of the same. These windings create a rotating magnetic field, whereas the rings form the armature of the electromagnetic system and can be either speeded up or slowed down relative to the rotating body by varying the frequency of the rotating magnetic field relative to the rotation frequency of the rotating body. Each ring has a stator winding of its own and the windings can be activated independently of each other, so that the rings can be moved separately of each other and in either way, which enables the right balance position of the whole structure to be reached more quickly.
These balancing devices by virtue of the fact that within the rotating part neither motors nor electronic components are present and actuation of the rings takes place without the presence of contacts, have very reduced sizes and can also advantageously be used for bodies rotating at very high speeds, 40,000/50,000 r.p.m. for example, enabling a quick and accurate balancing during rotation of these bodies, such as balancing of a tool/tool carrier assembly mounted in the spindle of a machine tool, for example.
The rings can be carried by respective bearings and be fastened to the outer rim of said bearings, whereas the inner rim is secured to the rotating body. It is however necessary to cause the rings to be driven in rotation by the rotating body. For this purpose, in a known solution permanent magnets are provided on the rings and pole plates are disposed in side by side relationship with the rings and rigidly connected to the rotating structure, so as to magnetically keep the rings and rotating body in a stable situation under normal rotating conditions. These known devices however have some drawbacks.
The presence of electric windings may involve interferences in the individual operation of the rings during the balancing step, because the magnetic field acting on one of the rings may affect the magnetic field acting on the other ring, which will make achievement of the right angular position for balancing of the rings more difficult. Therefore, some axial distance between the rings and the respective windings is to be maintained, but a greater axial distance may involve dynamic unbalances, because an undesirable torque may be generated in the axial plane of the rotating body due to the unbalanced rings spaced apart too much. Practically, a distance of at least 20 mm between the two rings is required, which leads to an overall axial bulkiness of about 40 mm.
Operation of the two rings one at a time is also possible but in this case more time is required for completing the balancing operation. A single winding may be also provided that is axially movable, so as to first act on one ring and then on the other, but this makes the device mechanically more complicated in addition to the fact that a longer period of time is required in order to obtain balancing. Otherwise, stator windings may be such arranged that each of them affects only one sector of the ring circumference and the windings are angularly offset so that the respective magnetic fields do not interfere with each other. With this arrangement, however, more powerful magnetic fields are necessary in order to carry out the angular displacements of the rings, and this brings about heavier costs.
Another drawback of these electromagnetically-driven devices is that they can magnetically attract swarf and other ferrous materials removed in machining, which will tend to accumulate in the regions where the magnetic field is present and to alter the field itself or form dangerous deposits, in particular at the air gap existing between the ferromagnetic core of the windings and the rings. These deposits can seriously jeopardize a good operation of the balancing device.
At all events these devices practically need a watertight protection in all cases in which sprays of cooling liquids exist that could otherwise create electric short circuits and damage the devices themselves.
In addition these devices can be hardly applied to existing machines.
Under this situation the present invention mainly aims at conceiving a balancing device for a rotating body, in particular for a tool carrier with a tool rotating at high speed, capable of obviating the above drawbacks.
Accordingly, it is an important aim of the invention to provided a balancing device of the type involving balancing rings, capable of executing a quick and accurate balancing of rotating bodies, even at rotation speeds as high as 40,000-50,000 r.p.m., with a structure of simple construction and minimum bulkiness.
It is another aim to provide a device of the above kind, enabling the axial bulkiness of the balancing rings to be maintained to a minimum, so as to avoid dynamic unbalances in the axial plane of the rotating body.
It is a further aim of the invention to provide a balancing device adapted to act on the balancing rings without the occurrence of mutual interference, even in the case of very close rings and of simultaneous operation of the rings.
Another aim of the invention is to provided a balancing device adapted to operate also in the presence of cooling liquids and great amounts of swarf or other removed materials resulting from machining operations, without this presence giving rise to malfunctions of the device.
A still further aim of the invention is to provide a device enabling a trouble-free positioning of same adjacent the balancing rings and adapted to be easily applied also to working machines already in use.
A further by no means last aim of the invention is to provide a device of the type specified above which is also cheap and therefore adapted for working machines and machining centers provided with a great number of tools, even different from each other.
The foregoing and further aims that will become more apparent in the following are achieved by a balancing device for a rotating body, in particular for a tool carrier with a tool rotating at high speed, comprising at least two annular balancing elements each having an unbalance and disposed in axial side by side relationship and coaxial with each other on said rotating body, said annular elements being rotatable with said body and adapted to be angularly rotated relative to said body by angular-positioning means associated with said annular elements, said angular-positioning means comprising means for generating a fluid flow directed onto said annular elements to temporarily and selectively speed up or slow down the rotation of said annular elements relative to said rotating body.